Click chemistry is a chemical philosophy proposed in 2001 (Kolb et al., 2001, Angew. Chem.-Int, Edit. 40(11):2004-24 whereby the chemistry is tailored to generate molecules quickly and reliably by joining smaller units together. The Click reaction paradigm is centered on the development and implementation of robust reactions that proceed with reliable control over the products formed.
The Click chemistry paradigm requires the following characteristics in the reactions under consideration: the reaction involves minimal set-up effort and the starting materials are readily available; the reaction is high yielding, proceeding with high stereospecificity and high atom economy; the reaction is run solvent-free or in a benign solvent (preferably water); the product can be easily isolated by crystallization or distillation, preparative chromatography not being required; the by-products are easily removed and non-toxic; the reaction is physiologically compatible; and there is a large thermodynamic driving force (>84 kJ/mol) to favor the formation of a single reaction product. It is unlikely that any reaction will meet all these criteria for every situation. However, several reactions have been identified as generally meeting all these criteria.
One such reaction is the azide-alkyne Huisgen cycloaddition, which is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole (Huisgen, 1961, Proc. Chem. Soc. London:357), Once described as “the cream of the crop” of Click chemistry, this reaction is arguably the most prolific and powerful of them (Kolb et al., 2001, Angew, Chem.-Int. Edit. 40(11):2004-24

A notable variant of the Huisgen 1,3-dipolar cycloaddition is the copper(I) catalyzed (or Cu(I)-catalyzed) variant, in which organic azides and terminal alkynes are united to afford 1,4-regioisomers of 1,2,3-triazoles as sole products. The Cu(I)-catalyzed variant was first reported for solid phase synthesis of peptidotriazoles (Tornøe et al., 2002, J. Org. Chem. 67:3057-64). While the Cu(I)-catalyzed variant gives rise to a triazole from a terminal alkyne and an azide, formally it is not a 1,3-dipolar cycloaddition and thus should not be termed a Huisgen cycloaddition. This reaction is better termed the Cu(I)-catalyzed Azide-Alkyne Cycloaddition (CuAAC).
CuAAC is ubiquitous and highly efficient in an ever increasing number of synthetic methodologies and applications, including bioconjugation (Wang et al., 2003, J. Am. Chem., Soc. 125(11)3192; El-Sagheer & Brown, 2010, Chem. Soc. Rev. 39(4):1388), labeling (Macpherson et al., 2007, Nature 445(7127):541; Cohen et al., 2007, Nat. Chem. Biol. 3(3):156), surface functionalization (Spruell et al., 2008, Angew. Chem.-Int. Edit, 47(51):9927), dendrimer synthesis (Peng et al., 2004, Angew. Chem.-Int. Edit. 43(30):3928), polymer synthesis (DeForest et al., 2009, Nat. Mat. 8(8):659), and polymer modification (Matyjaszewski & Tsarevsky, 2009, Nat. Chem. 1(4):276). The diverse implementation of the CuAAC reaction is due to its simplicity, capability of high yield, fast reaction kinetics, orthogonal reactivity, and tolerance to a wide variety of solvent conditions.
The CuAAC reaction may be run in a variety of solvents, such as aqueous solvents and (partially or fully) miscible organic solvents. The starting reagents need not be completely soluble for the reaction to be successful, and in many cases the product may be simply filtered from the solution as the only purification step required.
The CuAAC reaction may be performed using commercial sources of Cu(I) such as cuprous bromide or iodide, or in situ sources of Cu(I), such as a mixture of Cu(II) (e.g. copper(II) sulfate) and a reducing agent (e.g. sodium ascorbate). Since the Cu(I) catalyst is either directly added to the reaction bulk or generated by chemical reduction of Cu(II), temporal and/or spatial control of the CuAAC reaction is extremely limited at this point. While this deficit may seem trivial from a purely synthetic point of view, such control is necessitated in the creation of numerous functional materials (Bowman & Kloxin, 2008, AICHE J. 54(11):2775; Hoyle & Bowman, 2010, Angew. Chem.-Int. Edit. 49(9):1540) such as inks, coatings, adhesives, metamaterials, contact lenses, dental materials, and photoresists, and is employed in techniques such as parallel protein synthesis (Fodor et al., 1991, Science 251(4995):767), cell encapsulation (Koh et al., 2002, Langmuir 18(7):2459), tissue engineering (Martens et al., 2003, Biomacromol. 4(2):283), and 3D prototyping (Neckers, 1992, Polym. Eng. Sci. 32(20):1481; Young et al., 1999, J. Mina Sci. Eng.-Trans. ASME 121(3):474).
The CuAAC reaction mechanism is understood as involving three general steps: (I) the Cu(I)-acetylide formation, (II) the formal cycloaddition, and (III) catalyst regeneration (FIG. 1A) (Rodionov et al., 2007, JACS 129:12705-12712; Rostovtsev et al., 2002, Angew. Chem.-Int. Edit. 41:2596-2599; Himo et al., 2005, JACS 127:210-216). Under saturated conditions, where catalyst turnover is not required, the CuAAC reaction shows first order dependence on both azide and alkyne concentrations, consistent with an elementary bimolecular reaction (Rodionov et al., 2005, Angew. Chem.-Int. Edit, 44:2210-2215). In contrast, the kinetics are highly dependent on any ligands, buffer, salts, and substrates present when catalytic concentrations of Cu(I) are utilized (Rodionov et al., 2007, JACS 129:12705-12712). This behaviour has been interpreted as the consequence of a diverse family of copper coordination complexes that are formed in situ (Rodionov et al., 2007, JACS 129:12705-12712). In general, these copper species are highly reactive as evidenced by the consistent copper catalysis of the reaction for a large variety of terminal alkynes, but the reactivity of these species varies subtly to influence which step is rate-limiting.
Cu(I) is typically generated using sodium ascorbate as a reductant, which is used in large excess (10:1) to compensate for oxidation and disproportionation of Cu(I) (Himo et al., 2005, JACS 127:210-216; Chan et al., 2004, Org. Lett. 6:2853-2855). As such, nearly quantitative reduction of Cu(II) is assumed to occur. It is perhaps surprising, given the CuAAC reaction's susceptibility to other species, that the ascorbate anion appears to have no effect on the reaction kinetics (Rodionov et al., 2007, JACS 129:12705-12712). The radical mediated reduction of Cu(II) to copper metal has recently attracted renewed interest for both removal of hazardous wastes and generation of copper nanoparticles (Litter, 1999, Appl. Catal. B-Environ. 23:89-114; Sakamoto et al., 2008, Chem. Mater. 20:2060-2062). Due to Cu(II)/Cu(I)'s half reaction (reduction) potential of 0.16 V, a variety of organic radicals are capable of reducing it, e.g. ketyl, phosphinoyl, and semi-pinacol radicals generated by common photoinitiators such as bis(acyl)phosphines, α-hydroxyl ketones, and benzophenone. The reaction is typically described as occurring via the reduction of Cu(II) to Cu(I), followed by subsequent reduction to copper metal (reactions R1 and R2, respectively in FIG. 1A) (Kateda et al., 1968, Bull. Chem. Soc. Jpn. 41:268; Sakamoto et al., 2009, J. Photochem. Photobiol. C-Photochem. Rev. 10:33-56). Besides further reduction to copper metal, Cu(I) is both air sensitive and prone to disproportionation (reaction D in FIG. 1A) (Simmons et al., 1980, J. Chem. Soc.-Dalton Trans. 1827-1837). It would be expected that all three of these reactions could play a complicated role in the photo-mediated catalysis of the CuAAC reaction.
The critical need for complete spatial control of the CuAAC reaction is demonstrated by the extent to which researchers have gone to facilitate partial control of this reaction. Dip-pen lithography, with a Cu(I) inked tip or a copper tip (Long et al., 2007, Adv. Mater. 19(24):4471; Paxton et al., 2009, J. Am. Chem. Soc. 131(19):6692), has been used to trace a pattern on the substrate, and catalyze the CuAAC reaction between a functionalized surface, and the alternate Click reagent in the bulk. This technique has produced features as small as 50 nm, and micron scale features require one half second to complete. Similarly, microcontact printing utilizes an elastomeric stamp inked with a solution of reagent to promote spatial control (Rozkiewicz et al., 2006, Angew. Chem.-Int. Edit. 45(32):5292). The catalytic Cu(I) was either included in the solution, or generated from a copper metal layer coating the surface of the stamp. The stamp was brought into contact with a functionalized surface for minutes to an hour. Features on the order of tens of microns were fabricated using this technique. Electroclick chemistry utilizes an electric potential applied across a pair of electrodes (Devaraj et al., 2006, J. Am. Chem. Soc. 128(6):1794; Hansen et al., 2009, Adv. Mater. 21(44):4483). At the negative electrode Cu(II) was reduced to Cu(I) and the CuAAC reaction was subsequently catalyzed where features as small as 10 microns have been produced. These techniques unfortunately had drawbacks. Microcontact printing utilizes inexpensive elastomeric stamps than can rapidly reproduce images. However, the master stamps must be fabricated by another technique that is capable of directly writing the master. Electroclick chemistry shares this drawback. Dip-pen lithography is capable of directly writing high fidelity images, but is accordingly ill-suited to the reproduction of large images and features.
Alternatively, photolithography utilizes masked or focused light to irradiate a specific area, and induce chemical reactions that change the solubility of the photosensitive material. An image is then developed by immersion in a solvent. Photolithography may be directly used to both write images and reproduce images, even utilizing masks produced by inkjet printers (Qin et al., 1996, Adv. Mater. 8(11):917). Photolithography may also be used to produce three-dimensional images and reactions (Neckers, 1992, Polym. Eng. Sci. 32(20):1481; Young et al., 1999, J. Manuf. Sci. Eng.-Trans. ASME 121(3):474), to write images within a material (DeForest et al., 2009, Nat. Mat. 8(8):659) and to functionalize a material anywhere throughout the material. There have been no reports of generation of polymeric network-forming materials via CuAAC crosslinking reactions by conventional photolithographic methods.
Photochemical reactions have long attracted attention from synthetic chemists because these reactions allow the assembly of complex systems under mild conditions. Upon irradiation, photochemical reactions occur either by direct excitation of chromophoric species, as in the case of [2+2] cycloadditions, or by the generation of an active species that initiates multiple reactions, as typical of photopolymerizations. Photochemical 1,3-cycloadditions are symmetry forbidden (Padwa, 1976, Angew. Chem.-Int. Edit. Engl 15(3):123) and the photopatterning of azides and alkynes has been limited to the photochemical decarbonylation of propenones to dibenzocyclooctynes, which subsequently undergo the thermal 1,3-dipolar cycloaddition (copper-free azide-alkyne Click reaction) (Poloukhtine et al., 2009, J. Am. Chem. Soc. 13 (43):15769). Despite the success of this innovative approach for labeling cells (Poloukhtine et al., 2009, J. Am. Chem. Soc. 13 (43):15769) and functionalizing surfaces (Orski et al., 2010, J. Am. Chem. Soc. 13(32)11024), this approach is limited by the complexity and scarcity of materials possessing the requisite cyclopropenone functional group, the synthesis of which is non-trivial. Furthermore, this approach requires large irradiation doses to obtain high conversions, as each absorbed photon leads to a maximum of one bimolecular coupling event. In contrast, unprotected, reactive azides are readily synthesized from acyl halides, and typical photoinitiated polymerizations follow a chain reaction mechanism, where a single absorbed photon leads to a reaction cascade that ultimately consumes many reactant molecules per absorbed photon.
There is a need in the art to develop a novel method of catalyzing the CuAAC reaction that can be spatially and temporally controlled. This robust and controllable reaction would be a much needed addition to the repertoire of synthetic transformations available to synthetic chemists and material scientists. This reaction would find application not only in conventional small molecule synthesis but in surface modification, polymerization reactions and polymer modification reactions as well. The present invention fulfills this need.